Unit Affiliation: Geochemistry, Lamont-Doherty Earth Observatory (LDEO)
Measurements of water in the mineral olivine and its melt inclusions provide constraints on fundamental Earth processes such as the occurrence of plate tectonics, planetary melting, and volcanic explosivity. However, interpretations of these measurements are highly debated because of the possibility that water may exchange rapidly with the surrounding environment during transport from the earth's interior to the site of collection. The extent of this exchange depends on how fast water diffuses in olivine. Water diffusion in olivine can occur by several different mechanisms, some of which are orders of magnitude faster than others at the same temperature, resulting in considerable uncertainty about which diffusion rate to apply. This project aims to resolve that ambiguity while supporting an early-career female scientist; undergraduate students will also contribute to the investigation. The research results will be shared with the general public and the research community. This project will include targeted dehydration experiments as well as measurements of natural samples to elucidate the role of different diffusion mechanisms as a function of oxygen fugacity, temperature, and time. Oriented single crystals of olivine will be first completely dehydrated and equilibrated at the temperature of interest, then fully hydrated at high pressure, and finally placed in a 1-atm gas-mixing furnace and heated in steps until fully dehydrated. After each step, infrared spectroscopy (FTIR) measurements will be taken along profiles in each dimension of the sample to quantify the internal water concentration and extent of diffusion. This approach takes advantage of a new numerical technique for interpreting infrared spectroscopic measurements through uncut crystal blocks, allowing for the first time a time-series of hydrogen diffusion data on the same sample in multiple orientations. These laboratory experiments will be complemented by FTIR measurements of various sized olivine phenocrysts from Fuego Volcano. This study will be the first attempt to identify and apply site-specific diffusivities in samples that are well-characterized for their degassing and ascent history.